ACQUISITIONS - Narrative (Details) - USD ($) $ in Millions				12 Months Ended
	Jan. 06, 2015	Aug. 31, 2014	Sep. 01, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Business Acquisition [Line Items]
Acquisition of MGE				$ 0.0	$ (23.9)	$ 975.0
Proceeds from final reconciliation of acquisition of Missouri Gas Energy				0.0	11.0	$ 0.0
Goodwill				946.0	937.8
Missouri Gas Energy (MGE)
Business Acquisition [Line Items]
Acquisition of MGE			$ 975.0
Proceeds from subsequent reconciliation of net assets transferred in acquisition			23.9
Reconciliation of net assets transferred					940.2
Goodwill			210.2
Goodwill, expected to be tax deductible			$ 177.2
Alagasco
Business Acquisition [Line Items]
Goodwill		$ 735.8		$ 735.8	$ 727.6
Goodwill, expected to be tax deductible		$ 717.6
Percentage of voting interest acquired		100.00%
Payments to acquire businesses, net of cash acquired	$ (1,313.4)	$ (1,305.2)
Final reconciliation of net assets payments to acquire businesses	$ (8.2)